chapter - Atmospheric and Oceanic Science

a. Pre-industrial experiment:
Initial conditions for this experiment were derived from a multi-step process.
First, an atmosphere plus land surface model was started with forcing agents at levels
representative of the late 20th century. Second, the spun up atmosphere plus land
surface model was coupled to an ocean plus sea ice model. Initial conditions for sea
ice, ocean potential temperature and salinity were derived from late 20th century
observations. Forcing agents consistent with year 1860 were applied to the coupled
model which was integrated for a ~300 year adjustment period. Year 1 experiment
data begins at the end of this adjustment period. The forcing agents representative of
conditions of 1860 include the well-mixed greenhouse gases (CO2, CH4, N2O), tropospheric
and stratospheric O3, tropospheric sulphates, black and organic carbon,
dust, sea salt, solar irradiance, and the distribution of land cover types.
In the pre-industrial run with no anthropogenic forcing, we can consider the
variations due to the climate natural variability. Figure 13.10a shows the percentage
of annual climatological precipitation, between two periods (year 11 to year 70)
and (year 71 to year 280). It is seen very small differences over South America and
almost no change over the La Plata basin. Low values are also seen over the tropical
Pacific and Atlantic. There are not changes in the temperature field of the two
periods (Fig. 13.10b).
b.1% increase in CO2 up to doubling.
Regional climatic scenarios
The initial data correspond to 1 January of year 1 of the 1860 control model
experiment, which had CO2 concentration of 286.05 ppm. The increase rate is 1%
/year up to year 70 (when the CO2 =2 X initial CO2). Then, this concentration
(572.10 ppm) is held constant from year 71 to year 280. For the entire 280-year
duration of the experiment, all non-CO2 forcing agents (CH4, N2O, halons, tropospheric
and stratospheric O3, tropospheric sulphates, black and organic carbon, dust,
sea salt, solar irradiance, and the distribution of land cover types) were held constant
at values representative of year 1860.
Considering the annual mean of the same two periods discussed in the previous
analysis of the pre-industrial run, the impact of the CO2 increase on the South
America precipitation and nearby oceans is seen in figure 13.11. In the southern
sector of the La Plata basin there is a maximum of 20% increase in precipitation,
and in the northern sector of the basin, a maximum reduction of 10% precipitation.
In the northern sector the annual mean reflects the behaviour of the summer rainy
season. It is noticed the impact on the SACZ and Northeast Brazil, with less precipitation
in the second period near the coast and more precipitation over western
Amazonia region. There are large differences over tropical Pacific (increased precipitation)
that affects the Peruvian coast, and Southwest Pacific (reduced precipitation)
that affects Chile.
176